This application requests continued support for basic and clinical studies of the human plasma protein C pathway, a pathway that provides one of the body's major natural anticoagulant mechanisms. The long-term objective of the principal investigator is to study molecules and mechanisms that regulate thrombosis and hemostasis. Work supported by this grant led to discovery of homozygous and heterozygous protein C deficiency and protein S deficiency associated with hereditary venous thrombotic disease. Other work showed that inactivation of Factors V and VIII in plasma and neutralization of plasminogen activator inhibitor by activated protein C explain its anticoagulant and profibrinolytic properties. Protein S and protein S binding protein (PSBP) are cofactor enhancing activated protein C activity. The specific aims of this grant are concerned with defining further the mechanisms of actions of activated protein C, protein S, and PSBP as anticoagulant and profibrinolytic agents and studying plasma and urinary protein C inhibitors that downregulate the protein C pathway by complexing with activated protein C and identifying the protein C gene defects responsible for its deficiency. Other aims include clinical studies of PSBP and protein C inhibitor to prove the hypotheses that hereditary PSBP deficiency is associated with thrombophilia, that plasma concentrations of PSBP:Protein S complexes determine the clinical tendency to thrombosis in protein S deficient patients, and that protein C inhibitor deficiency is associated with a bleeding diathesis. Moreover, functional and immunologic measurements of the protein C pathway proteins in over 400 patients with protein C or protein S deficiency as well as in other patient groups at high risk for venous thrombosis will be made in order to find correlations between levels of certain molecular species and thrombotic manifestations or protection from thrombosis. It is anticipated that further clinical insights will follow new molecular insights and that information will be obtained concerning the potential utility of protein C, protein S, or PSBP as new therapeutic antithrombotic agents.